This study attempts to pharmacologically characterize axonal endings in the dorsal horn of the medulla (DHM). The methods employed include light and electron microscopic autoradiographic localization of labeled axonal endings in the DHM following application of tritiated (3H) putative neurotransmitters onto the DHM. The transmitters to be investigated include serotonin, norepinephrine and GABA. Our approach relies on the ability of neurons which normally utilize a specific neurotransmitter to take up this (3H) transmitter at their axonal endings. The purpose of these studies is to locate and morphologically characterize pharmacologically distinct axonal endings in the DHM which might be involved in trigeminal pain pathways.